The field of the invention is magnetic resonance angiography ("MRA"), and particularly, studies of the human vasculature which use contrast agents to enhance NMR signals for a portion of the exam.
Diagnostic studies of the human vasculature have many medical applications. X-ray imaging methods such as digital subtraction angiography ("DSA") have found wide use in the visualization of the cardiovascular system, including the heart and associated blood vessels. One of the advantages of these x-ray techniques is that image data can be acquired at a high rate (i.e. high temporal resolution) so that a sequence of images may be acquired during injection of the contrast agent. Such "dynamic studies" enable one to select the image in which the bolus of contrast agent is flowing through the vasculature of interest. Images showing the circulation of blood in the arteries and veins of the kidneys, the neck and head, the extremities and other organs have immense diagnostic utility. Unfortunately, however, these x-ray methods subject the patient to potentially harmful ionizing radiation and often require the use of an invasive catheter to inject a contrast agent into the vasculature to be imaged. There is also the issue of increased nephro-toxicity and allergic reactions to iodinated contrast agents used in conventional x-ray angiography.
Magnetic resonance angiography (MRA) uses the nuclear magnetic resonance (NMR) phenomenon to produce images of the human vasculature. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR angiography (MRA) has been an active area of research. Techniques that have been proposed and evaluated include time-of-flight, phase contrast, and contrast-enhanced MRA. The first class, time-of-flight (TOF) techniques, consists of methods which use the motion of the blood relative to the surrounding tissue. The most common approach is to exploit the differences in magnetization saturation that exist between flowing blood and stationary tissue. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal moving blood and the low-signal stationary tissues.
MRA methods have also been developed that encode motion into the phase of the acquired signal as disclosed in U.S. Pat. No. 32,701. These form the second class of MRA techniques and are known as phase contrast (PC) methods. Currently, most PC MRA techniques acquire two images, with each image having a different sensitivity to the same velocity component. Angiographic images are then obtained by forming either the phase difference or complex difference between the pair of velocity-encoded images.
To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Excellent diagnostic images may be acquired using contrast-enhanced MRA if the data acquisition is properly timed with the bolus passage.
Contrast-enhanced MRA is a well-accepted MRA technique that benefits from the relatively short TR and TE (e.g., &lt;7 msecs. and &lt;2 msecs., respectively) made possible through use of high-performance gradient systems. To achieve such short TR and TE, high-performance gradient systems produce gradient amplitudes and rise times on the order of 22 mTesla/meter (mT/m) and 77 Tesla/meter/second (T/m/s), respectively. Older gradient systems, referred to as "standard gradient systems," produce more modest gradient amplitudes and rise times on the order of 10 mTesla/m and 17 T/m/s, respectively.
As more standard gradient systems have been replaced by high-performance gradient systems, contrast-enhanced MRA has become more available and clinically useful. Even so, contrast-enhanced MRA does not fulfill, by itself, all desired objectives in an MRA examination. For example, in the context of analyzing the carotid arteries, two-dimensional time-of-flight (2DTOF) imaging provides an ideal localizer scan for subsequent contrast-enhanced MRA examination.
Moreover, in a paper by J. Huston III et al. entitled, "Carotid Artery: Prospective Blinded Comparison of Two-Dimensional Time-of-Flight MR Angiography with Conventional Angiography and Duplex US" (Radiology 1993; 186:339-344), the authors disclose that 2DTOF images of the carotid artery obtained using standard gradients reliably show a complete signal loss (i.e., a signal void) when the diameter of the artery is stenosed by 60% or more, as measured with the reference standard of conventional X-ray digital subtraction angiography. This feature is clinically useful since major clinical trials have shown endarterectomy is superior to medical therapy for patients who are symptomatic with 70% diameter stenosis, or are asymptomatic with a 60% diameter stenosis.
Thus, in determining whether to pursue endarterectomy or medical therapy, reliable detection of arterial stenosis in the range of 60% or more is highly useful. Although the paper noted above by J. Huston III et al. states that 2DTOF imaging using standard gradients facilitates reliable detection of arterial stenosis in the range of 60% or more, many MRI systems in use today employ high-performance gradients. Accordingly, it is worthwhile considering whether 2DTOF imaging using high-performance gradients reliably detects the desired degree of arterial stenosis.
It has been observed that high-performance gradients can preserve signal through stenotic segments with 2DTOF imaging. Although the arterial vessel generally appears narrowed, stenosis in excess of 75-85% is typically required before the signal is lost--unlike 2DTOF imaging using standard gradients which results in signal voids in the presence of 60% or greater arterial stenosis.
Moreover, a recently identified phenomenon, referred to as "ballooning," can result in an underestimation of the degree of arterial stenosis depicted with 2DTOF imaging under high-performance gradients. Partial volume effects and vessel motion are presently believed to be the root causes of ballooning. Thus, the phenomenon may occur when flow through a high-grade arterial stenosis results in intense signal that is averaged over multiple adjacent voxels resulting in an erroneously large apparent lumen. Consequently, the analyst of the inflated lumen will likely diagnose a lower grade stenosis, which may result in a false-negative diagnosis.
Thus, while the 2DTOF imaging technique may be used as a localizing scan to supplement the contrast-enhanced MRA exam, using the former technique with high-performance gradients limits its sensitivity in accurately detecting arterial stenosis in the clinically useful range of 60% or greater.